Cellular radio communication systems are generally used as mobile phone systems due to their ability to cover wide service areas. In these cellular radio communication systems, a plurality of base stations are arranged in a service area separately, and these base stations form a multitude of cells (zones) for covering the entire service area without gaps. In these cells, mobile stations can communication with other parties by using base stations.
Businesses which offer this type of mobile communication system are preassigned specific frequency bands. In each cell of a radio communication system, communication is performed between the mobile stations and base stations by using communication channels in these frequency bands.
Recently, the CDMA (Code Division Multiple Access) system has received attention as a radio communication system between mobile stations and base stations. In such CDMA radio communication systems, mobile stations and base stations perform a spread spectrum process using spreading codes on the signals to be transmitted, and these spread signals are transmitted to the other party. Here, different spreading codes are assigned to a plurality of mobile stations which perform communications at the same time. Therefore, in CDMA radio communication systems, a plurality of mobile stations in the same cell or in a plurality of adjacent cells can use the same frequency for communicating radio signals with the base stations.
FIG. 7 shows an example of cell groups in a CDMA radio communication system. In this CDMA radio communication system, four kinds of frequency f1, f2, f3 and f4 are provided as communication frequencies for communications between the mobile stations and a base station in a cell. However, the use of these four communication frequencies does not necessarily have to be allowed in every cell. For example, it is possible to allow the use of the communication frequencies f1–f4 in places with heavy traffic such as in a city center, and to allow the use of only the communication frequency f1 in places with light traffic such as in outlying areas, then sequentially add the available communication frequencies in the order of communication frequencies f2, f3 and f4 as the need arises in response to increases in traffic.
FIG. 8 shows an example of a map of the system capacity in a case where the number of available communication frequencies in each cell is increased in going from the outlying areas toward the city center. In each cell, a plurality of communication channels in the same frequency band can be used by using spreading codes which differ by the mobile station, whereby large amounts of traffic can be handled. The amount of traffic capable of being handled at the same time, i.e. the system capacity of each cell, depends on the number of available frequencies in each cell. Therefore, if the required system capacity becomes larger in the city center and smaller in approaching the outlying areas, then the number of available communication frequencies in each cell should be made smaller in going from a city center toward outlying areas as shown in FIG. 8. In the example shown in FIG. 8, the same communication frequency f1 can be used over the entire area. In this case, there is no need to switch the communication frequency being used due to movement between cells, and it is sufficient to switch the spreading codes, so that communication interruptions can be minimized.
FIG. 9 shows an example of a plurality of communication frequency bands in a specific frequency band allotted to a certain business in a CDMA radio communication system.
These communication frequency bands are arranged upon the frequency axis at frequency band gaps of B1, B2, . . . The frequency band gaps B1, B2, . . . can be the same value. The mobile stations and base mobile station exchange radio signals with other parties by using one of these communication frequency bands.
However, if for example non-linear distortion or the leakage occurs when a radio signal in a certain communication frequency band is amplified and output by a transmission power amplifier in a mobile station, a leakage signal will appear in the communication frequency band adjacent to that communication frequency band (hereafter referred to as adjacent frequency band).
As shown in FIG. 10, the power of leakage signal is usually strongest in the adjacent frequency bands, and becomes extremely weak in the next adjacent frequency bands.
Leakage signals with large powers influence reception operations of mobile stations and base stations using the adjacent frequency bands. Here, the mobile stations and base stations have reception filters for attenuating such leakage signals from adjacent frequency bands. However, unless the attenuation properties outside the bands of the reception filters are sufficient, the influence of interference from the adjacent frequency bands (hereafter referred to as adjacent channel interference) becomes large.
This adjacent channel interference causes reductions in the reception sensitivity and intermixture of noise in the mobile stations and base stations. For this reason, a guard band G is provided for suppressing adjacent channel interference between the communication frequency bands as shown in FIG. 11.
The width of this guard band G affects the system capacity of the radio communication system.
More particularly, the theoretical system capacity S can be obtained by the following formula (1).S=C(W−KG)/D=CN   (1)
In the above formula (1), C denotes the number of communication channels capable of using the same communication frequency band, W denotes the bandwidth of the entire frequency band allotted to a mobile communication service business, K denotes the number of guard bands provided in this frequency band, G denotes the bandwidth of each guard band, and N denotes the number of communication frequency bands.
As shown in FIG. 12, the system capacity S can be made larger if the guard band G is narrower because the number N of the communication frequency bands can be made larger. However, if the guard band B is narrow, then the amount of adjacent channel interference becomes large. Furthermore, if the guard band G are too narrow, then the use of communication channels which are influenced by adjacent channel interference is restricted, so that the system capacities will be conversely be reduced.
On the other hand, if the guard bands G are made wide as shown in FIG. 13, it is possible to keep the adjacent channel interference small. However, in this case, the system capacity S will be reduced.
In order to suppress the adjacent channel interference and retain the necessary system capacity, it is necessary to provide ways of suppressing adjacent channel interference without depending on only the method of widening the widths of the guard bands G.
For this reason, conventional cellular radio communication systems suppress adjacent channel interference by performing transmission power control to keep the transmission output of the transmission power amplifier circuits of the mobile stations and base stations as low as possible within a range such as to maintain the necessary communication quality.
For example, in FIG. 14, when a mobile station is near a base station and the quality of reception signals from the mobile station in the base station is high, then the transmission power of the mobile station is made low. On the other hand, if the mobile station is far from the base station and the quality of the reception signals from the mobile station in the base station is low, then the transmission power of the mobile station is made high.
There are cases in which a plurality of businesses provide mobile communication services in the same or overlapping service areas. In this case, the businesses share the use of the frequency bands offered for those services. FIGS. 15(a) and (b) show examples thereof. First, in the example shown in FIG. 15(a), the frequency band offered for a mobile communication service is divided into three parts, and the divided frequency bands are allotted respectively to the businesses A, B and C. Additionally, in the example shown in FIG. 15(b), a frequency band offered for a mobile communication service is shared by the businesses A and B. Each business provides communication services using the allotted frequency band.
Here, transmission power control is performed in the base stations and mobile stations adapted for each business, as a result of which the effects of adjacent channel interference are minimized.
However, when a plurality of businesses offer mobile communication services in an overlapping service area, there are cases in which adjacent channel interference of a considerable interference level occurs between different businesses and this cannot be sufficiently suppressed even if transmission power control is performed by both the base stations and mobile stations adapted to the respective businesses.
Herebelow, a typical example wherein the problem of adjacent channel interference between different businesses occurs shall be explained.
In FIG. 16, the base station 20A is a communication installation of business A, and the base station 20B is a communication installation of business B. Additionally, the cell 10A is a cell formed by base station 20A and the cell 10B is a cell formed by base station 20B. Furthermore, the mobile station 30A is a mobile station of a user contracted with business A and the mobile station 30B is a mobile station of a user contracted with business B.
As shown in FIG. 16, the base station 20A of business A is located at the edge of the cell 10B formed by the base station 20B of business B. When the base stations of different businesses have this type of geographical relationship, adjacent channel interference of a larger interference level than that which occurs within the same business can occur.
Suppose a case in which the mobile station 30B is in communication using a communication channel adjacent to a frequency band allotted to business A as shown in FIG. 16.
In this case, the mobile station 30B may transmit at maximum power in the vicinity of the base station 20A of business A in order to make the transmission signal reach the base station 20B which is far away.
At this time, the uplink transmission signal transmitted by the mobile station 30B includes, in addition to a signal corresponding to the uplink channel allotted to the mobile station 30B, a leakage signal in a channel adjacent to this uplink channel, i.e. in the communication channel used by the base station 20A of business A. This leakage signal interferes with base station 20A of business A. The mobile station 30A which is present in cell 10A must then increase the transmission power in order to compensate for decreases in the communication quality due to the effects of this interference.
If the mobile station 30A moves to the edge of cell 10A as shown in FIG. 16, the mobile station 30A must raise the transmission power in accordance with the distance to the base station 20A.
However, as shown in FIG. 16, if the mobile station 30A is far from the base station 20A and the mobile station 30B which is outputting a leakage signal which causes adjacent channel interference is located in the immediate vicinity of the base station 20A, adjacent channel interference of an extremely high level will influence the communications between the mobile station 30A and the base station 20A.
In this case, even if the transmission power of the mobile station 30A is at maximum, there is a possibility that the effects of the adjacent channel interference due to the leakage signals from the mobile station 30A will not be able to be reduced.
Generally speaking the number of both mobile stations and base stations is large. Additionally, the mobile stations in a service area move arbitrarily.
Therefore, decreases in the system capacity due to adjacent channel interferences are manifested in increases in the proportion of parts of service areas in which a predetermined quality cannot be obtained or in increases in the proportion of time during which a predetermined quality cannot be obtained at the same location.
In order to gain the reliance of mobile communication services users, it is necessary to reduce the proportion of area or time during which this predetermined level of quality cannot be obtained.